The kinetic mechanism of an enzyme is determined in two parts: the order of addition of substrates and release of products and the location of slow steps along the reaction pathway. Techniques are available to obtain this information which include initial rate studies in the presence and absence of products and dead end inhibitors, isotope exchange at equilibrium, and isotope effect studies. More recent advances in steady state kinetic theory have shown that the chemical catalytic mechanism can also be obtained using a determination of the pH variation of primary and secondary isotope effects (particularly secondary effects which are more sensitive to transition state structure) and kinetic parameters (for example, Vmax, V/K, and Ki for a substrate or inhibitor). It is the goal of this project to determine the mechanism of action of enzymes which include malic enzyme from Ascaris suum (responsible for the oxidative decarboxylation of L-malate by DPN to pyruvate and CO2) using the approach outlined above. This kind of approach has been used with success for several enzymes including hexokinase and fructokinase but only a very small number of enzymes have mechanisms which are known with any degree of certainty.